I do have indeed a silver bounce umbrella so I'll definitely bring that along. Also no problem for WB as I do shoot raw.

I am thinking I will need some kind of separation to get some privacy as this is a common area where people come and go to get the elevator. Just to understand, would a white backdrop make things easier if placed opposite the window for getting the right light? (which would give me also that privacy in the room)

Also for this kind of professional looking portrait is it suggested the classic 45 degrees umbrella or can you also use other portrait techniques like butterfly, split light, broad, Rembrandt etc?